A lot of carbon materials derived from petroleum resources are still used today. However, exhaustion of the resources is becoming reality and novel carbon materials different from those derived from petroleum resources have been demanded. As an example of the novel carbon materials, an attempt has been made to use oils and fats and essential oils which are derived from natural plants, as a lubricating oil.
For instance, plant-derived fats and oils representative of soybean oil, wood oil, palm oil and the like can be obtained by expressing and extracting plants to provide crude oil and purifying the crude oil by distillation and the like. The plant-derived fats and oils are saturated or unsaturated fatty acid triglyceride and used in various fields such as cooking, fuels, medicine and lubricating oils. Moreover, an attempt to obtain a lubricating base oil exhibiting excellent low-temperature fluidity, oxidation stability and lubricity from the above plant-derived oils and fats according to an ester exchange method is also disclosed (see Patent Literature 1).
The plant-derived essential oils contain alcohols, aldehydes, ketones, esters, phenols, hydrocarbons and the like as components. Hydrocarbons are exemplified by terpenes and sesquiterpenes. Known examples of terpenes include: chain terpenes such as alloocimene, ocimene, myrcene and dihydromyrcene; and cyclic terpenes such as α-pinene, β-pinene, limonene, camphene, α-phellandrene, terpinene, terpinolene and 3-carene. Terpenes, which are often used in perfumes, are hydrocarbons having a molecular formula of C10H16. Since terpenes exhibit low viscosity, low flash point and the like although having high hydrolytic stability and volume resistivity, terpenes are not suitable for using as lubricating oils. Known examples of sesquiterpenes include: chain sesquiterpenes such as farnesene; and cyclic sesquiterpenes such as cedrene, β-caryophyllene, cadinene, valencene, Thujopsis and Guaiene. Sesquiterpenes are hydrocarbons having a molecular formula of C15H24. Since sesquiterpenes exhibit a high hydrolytic stability, a high volume resistivity, and further suitable viscosity and flash point, sesquiterpenes are usable as a low-viscous material for a lubricating oil.
In recent years, fuel efficiency regulations have been reinforced in each country because of growing awareness of environmental problems. In order to respond to such growing needs to improve a fuel efficiency, a continuously variable transmission (hereinafter, also referred to as CVT) tends to be employed. Since a speed is continuously variable in CVT, the most suitable engine speed can be selected corresponding to required output torque, resulting in a large improvement in the fuel efficiency. Moreover, since the engine speed is variable without a shock and is free from a fall in elevating a shift in CVT, an acceleration performance is improved and drivability is excellent. CVT is exemplified by a metal belt type CVT, a chain type CVT and a traction drive type CVT, each of which requires a high transmission efficiency. Accordingly, it is required to improve a transmission efficiency by developing a lubricating oil having a high traction coefficient. Above all, the traction drive type CVT is the most suitable for a high-class car since the traction drive type CVT transmits power through an oil film to make less noise.
On the other hand, since a lubricating oil for a continuously variable transmission is for transmitting power from a driving section to a driven section through an oil film, a higher traction coefficient of the lubricating oil results in a higher transmission efficiency. Accordingly, the traction coefficient is desired to be sufficiently large during an actual use.
Particularly, since the lubricating oil for the traction drive type CVT also serves as a typical lubricating oil in CVT, the lubricating oil for the traction drive type CVT needs to have a high viscosity enough to retain a sufficient oil film even at high temperatures for prevention of friction wear.
On the other hand, the lubricating oil for the traction drive type CVT needs to have a low viscosity at low temperatures (i.e., low-temperature fluidity) for a low-temperature starting performance in cold areas such as North America and North Europe. Accordingly, the lubricating oil for the traction drive type CVT requires a low dependency of a viscosity on temperatures, that is, a high viscosity index. In order to respond to such a demand, as shown in, for instance, Patent Literature 2, fluid for traction drive type CVT having a viscosity index of at least zero and containing a bicyclo[2.2.1]heptane derivative has been developed.